The main measurement quantities for semiconductor metrology are overlay (OVL), critical dimensions (CD) (which includes: mean critical dimension, height and side wall angle; thus characterizing the printed features profile) and focus-dose. There are two main approaches for optical metrology available: imaging and scatterometry. Overlay can be measured using either approach while critical dimensions (performed on device rule targets) can only be performed using scatterometry methods. Imaging indicates a method where the required metrics are extracted from some visualization of a certain target. The visualization is achieved by measuring light intensity in an optically conjugate plane to the target plane. Scatterometry is in essence, extraction of said quantities from information achieved from discrete or continuous diffraction orders from the target. These diffraction orders are usually viewable in an optically conjugate plane of the system objective pupil. The electric field in this pupil contains a truncated Fourier transform of the target, the truncation depending on the numerical aperture (angular acceptance) of the said objective. The continuous reduction of the target sizes and overlay control budgets require increasingly accurate metrological methods.
Imaging technologies include, but are not limited to: classical imaging as disclosed e.g., by U.S. Pat. No. 7,541,201, which is incorporated herein by reference in its entirety, and order selected imaging as disclosed e.g., by U.S. Pat. No. 7,528,941, which is incorporated herein by reference in its entirety.
FIGS. 1A-1C are schematic illustration of contours of pupil light distribution for different target sizes, according to the prior art. FIGS. 1A-C illustrate a main disadvantage of typical imaging technologies, based on an example suggested by U.S. Pat. No. 7,528,941, referred to above. These optical configurations are characterized by quasi-normal illumination and zero order blocking in the collection pupil. The target image is constructed by interference of the ±1 diffraction orders. Such a configuration provides a robust (i.e., independent of optical imperfections) OVL measurement with very high performance. The system resolution is given by P>λ/[NA·(1+σ)], where λ is the wavelength of light, NA is the system numerical aperture and σ is a partial coherence factor. For example, in the visual spectrum with a standard NA=0.7 and σ=0.5, the pitch limit is P>400 nm. FIGS. 1A, 1B, and 1C illustrate the 1% of maximum intensity contours 75 in the pupil plane for a grating having a 1000 nm pitch and imaged with λ=532 nm, for target sizes 27 μm, 15 μm and 10 μm, respectively. Circles 71 indicate the 0.7NA pupil region and circles 72 indicate the zero order blocking obscuration. Evidently, targets smaller than 10 μm cannot be measured using this technology since the limited pupil NA leads to truncation of the transferred signal (any light out of the 0.7NA region is truncated). As a result of this truncation, significant image distortion is observed. Since the current market requirement for target size is 5 μm, this optical technology cannot be used without significant increase of the pupil's NA.
Imaging based OVL measurements are performed using a conventional microscope which provides a far-field image of the OVL target on a CCD. There are various imaging targets, as disclosed e.g., by U.S. Pat. No. 7,317,824, which is incorporated herein by reference in its entirety.
Scatterometry technologies include, but are not limited to: spectroscopic scatterometry, as disclosed e.g., by U.S. Pat. No. 7,616,313, which is incorporated herein by reference in its entirety, and angle-resolved scatterometry, as disclosed e.g. by U.S. Pat. No. 7,656,528, which is incorporated herein by reference in its entirety.
FIG. 4A illustrates typical dielectric-metal-dielectric stacks 90. In such an application, there is difficulty in dampening an amplitude enhancement by the interface between dielectric layers 90A, 90C and the surrounding air.